Semiconductor device with air gap between gate-all-around transistors and method for forming the same

ABSTRACT

The present disclosure provides a semiconductor device with an air gap between gate-all-around (GAA) transistors and a method for forming the semiconductor device. The semiconductor device includes a first gate stack and a second gate stack disposed over a semiconductor substrate. At least one of the first gate stack and the second gate stack includes a plurality of gate layers, and the first gate stack and the second gate stack have an air gap therebetween. The semiconductor device also includes a first gate structure and a second gate structure disposed over the first gate stack and the second gate stack, respectively, and a first dielectric layer surrounds lower sidewalls of the first gate structure and lower sidewalls of the second gate structure. A width of the first gate structure is greater than a width of the first plug.

TECHNICAL FIELD

The present disclosure relates to a semiconductor device and a methodfor forming the same, and more particularly, to a semiconductor devicewith an air gap below a landing pad and a method for forming the same.

DISCUSSION OF THE BACKGROUND

Semiconductor devices are essential for many modern applications. Withthe advancement of electronic technology, semiconductor devices arebecoming smaller in size while providing greater functionality andincluding greater amounts of integrated circuitry. Due to theminiaturized scale of semiconductor devices, various types anddimensions of semiconductor devices providing different functionalitiesare integrated and packaged into a single module. Furthermore, numerousmanufacturing operations are implemented for integration of varioustypes of semiconductor devices.

However, the manufacturing and integration of semiconductor devicesinvolve many complicated steps and operations. Integration insemiconductor devices becomes increasingly complicated. An increase incomplexity of manufacturing and integration of the semiconductor devicemay cause deficiencies, such as poor electrical interconnection due tomisalignment between upper conductive features and lower conductivefeatures. Accordingly, there is a continuous need to improve themanufacturing process of semiconductor devices so that the problems canbe addressed.

This Discussion of the Background section is provided for backgroundinformation only. The statements in this Discussion of the Backgroundare not an admission that the subject matter disclosed in this sectionconstitutes prior art to the present disclosure, and no part of thisDiscussion of the Background section may be used as an admission thatany part of this application, including this Discussion of theBackground section, constitutes prior art to the present disclosure.

SUMMARY

In one embodiment of the present disclosure, a semiconductor device isprovided. The semiconductor device includes a first lower plug and asecond lower plug disposed over a semiconductor substrate. Thesemiconductor device also includes a first landing pad disposed over atop surface and upper sidewalls of the first lower plug, and a firstupper plug disposed over the first landing pad and electricallyconnected to the first lower plug. A width of the first lower plug isgreater than a width of the first upper plug. The semiconductor devicefurther includes a dielectric layer disposed over the semiconductorsubstrate. The first lower plug, the second lower plug, the firstlanding pad and the first upper plug are disposed in the dielectriclayer, and the dielectric layer includes an air gap disposed between thefirst lower plug and the second lower plug.

In an embodiment, the semiconductor device further includes a secondlanding pad disposed over a top surface and upper sidewalls of thesecond lower plug, and a second upper plug disposed over the secondlanding pad and electrically connected to the second lower plug, whereina distance between the first upper plug and the second upper plug isgreater than a distance between the first lower plug and the secondlower plug. In an embodiment, a bottom width of the first lower plug isgreater than a top width of the first lower plug, and the top width ofthe first lower plug is greater than the width of the first upper plug.In an embodiment, a top surface of the first lower plug is higher than atopmost surface of the air gap. In an embodiment, the first landing padis made of copper germanide (Cu₃Ge).

In an embodiment, the semiconductor device further includes a firstbarrier layer covering a bottom surface, lower sidewalls and the uppersidewalls of the first lower plug, wherein a portion of the firstbarrier layer is sandwiched between the first landing pad and the firstlower plug. In an embodiment, the first landing pad includes an innerportion covering the top surface of the first lower plug, and aplurality of outer portions covering top surfaces and upper sidewalls ofthe first barrier layer, wherein the inner portion and the outerportions are made of different materials. In an embodiment, top surfacesof the outer portions of the first landing pad are higher than a topsurface of the inner portion of the first landing pad. In an embodiment,the inner portion of the first landing pad is made of tungsten silicide(WSi), and the outer portions of the first landing pad are made oftitanium silicide (TiSi).

In another embodiment of the present disclosure, a semiconductor deviceis provided. The semiconductor device includes a first gate stack and asecond gate stack disposed over a semiconductor substrate. At least oneof the first gate stack and the second gate stack includes a pluralityof gate layers, and the first gate stack and the second gate stack havean air gap therebetween. The semiconductor device also includes a firstgate structure and a second gate structure disposed over the first gatestack and the second gate stack, respectively, and a first dielectriclayer surrounding lower sidewalls of the first gate structure and lowersidewalls of the second gate structure. The semiconductor device furtherincludes a first landing pad disposed over a top surface and uppersidewalls of the first gate structure, and a first plug disposed overthe first landing pad and electrically connected to the first gatestructure. A width of the first gate structure is greater than a widthof the first plug. In addition, the semiconductor device includes asecond dielectric layer disposed over the first dielectric layer. Thefirst landing pad and the first plug are surrounded by the seconddielectric layer.

In an embodiment, the semiconductor device further includes a secondlanding pad disposed over a top surface and upper sidewalls of thesecond gate structure, and a second plug disposed over the secondlanding pad and electrically connected to the second gate structure,wherein a distance between the first plug and the second plug is greaterthan a distance between the first gate stack and the second gate stack.In an embodiment, the first landing pad is in direct contact with thefirst dielectric layer. In an embodiment, the semiconductor devicefurther includes a semiconductor material disposed between the firstgate stack and the second gate stack, wherein the air gap is enclosed bythe semiconductor material. In an embodiment, the first landing pad ismade of copper germanide (Cu₃Ge).

In an embodiment, the semiconductor device further includes a firstbarrier layer covering a bottom surface, the lower sidewalls and theupper sidewalls of the first gate structure, wherein a portion of thefirst barrier layer is sandwiched between the first landing pad and thefirst gate structure. In an embodiment, the first landing pad includesan inner portion covering the top surface of the first gate structure,and a plurality of outer portions covering top surfaces and uppersidewalls of the first barrier layer, wherein the inner portion and theouter portions are made of different materials. In an embodiment, topsurfaces of the outer portions of the first landing pad are higher thana top surface of the inner portions of the first landing pad. In anembodiment, the inner portion of the first landing pad is made oftungsten silicide (WSi), and the outer portions of the first landing padare made of titanium silicide (TiSi).

In another embodiment of the present disclosure, a method for forming asemiconductor device is provided. The method includes forming a firstlower plug and a second lower plug over a semiconductor substrate, andforming a first dielectric layer surrounding the first lower plug andthe second lower plug. A top surface and upper sidewalls of the firstlower plug and a top surface and upper sidewalls of the second lowerplug are protruded from the first dielectric layer. The method alsoincludes performing a heat treatment process to form a first landing padover the top surface and the upper sidewalls of the first lower plug anda second landing pad over the top surface and the upper sidewalls of thesecond lower plug, and removing the first dielectric layer after theheat treatment process is performed such that an opening is formedbetween the first lower plug and the second lower plug. The methodfurther includes depositing a second dielectric layer in the opening andover the first landing pad and the second landing pad such that an airgap is formed in the opening and enclosed by the second dielectriclayer, and forming a first upper plug in the second dielectric layer andover the first landing pad. A width of the first lower plug is greaterthan a width of the first upper plug.

In an embodiment, the method further includes forming a second upperplug in the second dielectric layer and over the second landing pad,wherein a distance between the first upper plug and the second upperplug is greater than a distance between the first lower plug and thesecond lower plug. In an embodiment, a silicon-containing gas is usedduring the heat treatment process. In an embodiment, the method furtherincludes forming a sacrificial dielectric layer with a first opening anda second opening over a semiconductor substrate, and forming a firstbarrier layer and a second barrier layer lining the first opening andthe second opening, respectively. In addition, the method includesforming the first lower plug and the second lower plug over the firstbarrier layer and the second barrier layer, respectively, and removingthe sacrificial dielectric layer after the first lower plug and thesecond lower plug are formed.

In an embodiment, the first opening and the second opening have taperedprofiles tapering away from the semiconductor substrate. In anembodiment, the first landing pad includes a first inner portioncovering the top surface of the first lower plug, and a plurality ofouter portions covering top surfaces and upper sidewalls of the firstbarrier layer, wherein top surfaces of the outer portions are higherthan a top surface of the inner portion, and wherein the inner portionand the outer portions are formed simultaneously by the heat treatmentprocess.

In another embodiment of the present disclosure, a method for forming asemiconductor device is provided. The method includes forming a firstgate stack and a second gate stack over a semiconductor substrate. Atleast one of the first gate stack and the second gate stack includes aplurality of gate layers, and the first gate stack and the second gatestack have an air gap therebetween. The method also includes forming afirst gate structure and a second gate structure over the first gatestack and the second gate stack, respectively, and forming a firstdielectric layer surrounding the first gate structure and the secondgate structure. A top surface and upper sidewalls of the first gatestructure and a top surface and upper sidewalls of the second gatestructure are protruded from the first dielectric layer. The methodfurther includes performing a heat treatment process to form a firstlanding pad over the top surface and the upper sidewalls of the firstgate structure and a second landing pad over the top surface and theupper sidewalls of the second gate structure. In addition, the methodincludes forming a second dielectric layer covering the first landingpad and the second landing pad, and forming a first plug in the seconddielectric layer and over the first landing pad. A width of the firstgate structure is greater than a width of the first plug.

In an embodiment, the method further includes forming a second plug inthe second dielectric layer and over the second landing pad, wherein adistance between the first plug and the second plug is greater than adistance between the first gate stack and the second gate stack. In anembodiment, a silicon-containing gas is used during the heat treatmentprocess. In an embodiment, the method further includes forming asacrificial dielectric layer with a first opening and a second openingover the first gate stack and the second gate stack, and forming a firstbarrier layer and a second barrier layer lining the first opening andthe second opening, respectively. In addition, the method includesforming the first gate structure and the second gate structure over thefirst barrier layer and the second barrier layer, respectively, andremoving the sacrificial dielectric layer after the first gate structureand the second gate structure are formed.

In an embodiment, the first landing pad includes a first inner portioncovering the top surface of the first gate structure, and a plurality ofouter portions covering top surfaces and upper sidewalls of the firstbarrier layer, wherein top surfaces of the outer portions are higherthan a top surface of the inner portion, and wherein the inner portionand the outer portions are formed simultaneously by the heat treatmentprocess.

Embodiments of a semiconductor device and method for forming the sameare provided in the disclosure. In some embodiments, the semiconductordevice includes a plurality of lower plugs disposed over a semiconductorsubstrate (or a plurality of gate structures over gate stacks when thesemiconductor device is a gate-all-around (GAA) transistor), a landingpad disposed over a top surface and upper sidewalls of one of the lowerplugs, and an upper plug disposed over the landing pad. The landing padprovides an increased landing area for the upper plug to land on.Therefore, the misalignment issues between the upper plug and the lowerplug may be prevented or reduced. Moreover, an air gap is formed belowthe landing pad and between the lower plugs (or between the gate stackswhen the semiconductor device is a GAA transistor). Therefore, theparasitic capacitance between the lower plugs (or the gate stacks) maybe reduced. As a result, the yield rate of the semiconductor device maybe increased, and the overall device performance may be improved.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription of the disclosure that follows may be better understood.Additional features and advantages of the disclosure will be describedhereinafter, and form the subject of the claims of the disclosure. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present disclosure. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the disclosure as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a cross-sectional view illustrating a semiconductor device, inaccordance with some embodiments.

FIG. 2 is a cross-sectional view illustrating a semiconductor device, inaccordance with some embodiments.

FIG. 3 is a cross-sectional view illustrating a semiconductor device, inaccordance with some embodiments.

FIG. 4 is a cross-sectional view illustrating a semiconductor device, inaccordance with some embodiments.

FIG. 5 is a flow diagram illustrating a method for forming asemiconductor device, in accordance with some embodiments.

FIG. 6 is a cross-sectional view illustrating an intermediate stage offorming lower plugs during the formation of the semiconductor device, inaccordance with some embodiments.

FIG. 7 is a cross-sectional view illustrating an intermediate stage offorming a first dielectric layer surrounding the lower plugs during theformation of the semiconductor device, in accordance with someembodiments.

FIG. 8 is a cross-sectional view illustrating an intermediate stage offorming landing pads over the lower plugs during the formation of thesemiconductor device, in accordance with some embodiments.

FIG. 9 is a cross-sectional view illustrating an intermediate stage ofremoving the first dielectric layer during the formation of thesemiconductor device, in accordance with some embodiments.

FIG. 10 is a cross-sectional view illustrating an intermediate stage ofdepositing a second dielectric layer such that an air gap is formedbetween the lower plugs during the formation of the semiconductordevice, in accordance with some embodiments.

FIG. 11 is a flow diagram illustrating a method for forming asemiconductor device, in accordance with some embodiments.

FIG. 12 is a cross-sectional view illustrating an intermediate stage offorming gate structures over gate stacks during the formation of thesemiconductor device, in accordance with some embodiments.

FIG. 13 is a cross-sectional view illustrating an intermediate stage offorming a first dielectric layer surrounding the gate structures duringthe formation of the semiconductor device, in accordance with someembodiments.

FIG. 14 is a cross-sectional view illustrating an intermediate stage offorming landing pads over the gate structures during the formation ofthe semiconductor device, in accordance with some embodiments.

FIG. 15 is a flow diagram illustrating a method for forming asemiconductor device, in accordance with some embodiments.

FIG. 16 is a cross-sectional view illustrating an intermediate stage offorming a sacrificial dielectric layer with openings exposing asemiconductor substrate during the formation of the semiconductordevice, in accordance with some embodiments.

FIG. 17 is a cross-sectional view illustrating an intermediate stage offorming barrier layers and lower plugs in the openings during theformation of the semiconductor device, in accordance with someembodiments.

FIG. 18 is a cross-sectional view illustrating an intermediate stage ofremoving the sacrificial dielectric layer during the formation of thesemiconductor device, in accordance with some embodiments.

FIG. 19 is a cross-sectional view illustrating an intermediate stage offorming a first dielectric layer surrounding the lower plugs and thebarrier layers during the formation of the semiconductor device, inaccordance with some embodiments.

FIG. 20 is a cross-sectional view illustrating an intermediate stage offorming landing pads over the lower plugs and the barrier layers duringthe formation of the semiconductor device, in accordance with someembodiments.

FIG. 21 is a cross-sectional view illustrating an intermediate stage ofremoving the first dielectric layer during the formation of thesemiconductor device, in accordance with some embodiments.

FIG. 22 is a flow diagram illustrating a method for forming asemiconductor device, in accordance with some embodiments.

FIG. 23 is a cross-sectional view illustrating an intermediate stage ofdepositing a second dielectric layer such that an air gap is formedbetween the lower plugs during the formation of the semiconductordevice, in accordance with some embodiments.

FIG. 24 is a cross-sectional view illustrating an intermediate stage offorming a sacrificial dielectric layer with openings exposing gatestacks during the formation of the semiconductor device, in accordancewith some embodiments.

FIG. 25 is a cross-sectional view illustrating an intermediate stage offorming barrier layers and lower plugs in the openings during theformation of the semiconductor device, in accordance with someembodiments.

FIG. 26 is a cross-sectional view illustrating an intermediate stage ofremoving the sacrificial dielectric layer during the formation of thesemiconductor device, in accordance with some embodiments.

FIG. 27 is a cross-sectional view illustrating an intermediate stage offorming a first dielectric layer surrounding the gate structures and thebarrier layers during the formation of the semiconductor device, inaccordance with some embodiments.

FIG. 28 is a cross-sectional view illustrating an intermediate stage offorming landing pads over the gate structures and the barrier layersduring the formation of the semiconductor device, in accordance withsome embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIG. 1 is a cross-sectional view illustrating a semiconductor device100, in accordance with some embodiments. In some embodiments, thesemiconductor device 100 includes a semiconductor substrate 101, aplurality of lower plugs 133 disposed over the semiconductor substrate101, a plurality of landing pads 137 disposed over the lower plugs 133,and a plurality of upper plugs 153 disposed over the landing pads 137.In some embodiment, the upper plugs 153 are electrically connected tothe lower plugs 133 through the landing pads 137. In addition, thesemiconductor device 100 includes a dielectric layer 143 (also referredto as inter-metal dielectric (IMD) layer) disposed over thesemiconductor substrate 101.

In some embodiments, the lower plugs 133, the landing pads 137 and theupper plugs 153 are disposed in the dielectric layer 143. In otherwords, the lower plugs 133, the landing pads 137 and the upper plugs 153are surrounded by the dielectric layer 143. Moreover, the dielectriclayer 143 includes an air gap 150 disposed between the adjacent lowerplugs 133. In some embodiments, the air gap 150 is enclosed by thedielectric layer 143. In addition, it should be noted that in someembodiments, the landing pads 137 are disposed over the top surfaces andthe upper sidewalls of the lower plugs 133. Specifically, the topsurfaces and the upper sidewalls of the lower plugs 133 are covered byand in direct contact with the landing pads 137.

FIG. 2 is a cross-sectional view illustrating a semiconductor device200, in accordance with some embodiments. In some embodiments, thesemiconductor device 200 includes a semiconductor substrate 201 whichhas been patterned to form a plurality of gate stacks 205 a and 205 b.At least one of the gate stacks 205 a and 205 b has a plurality ofalternating stacked semiconductor layers (semiconductor substrate 201)and gate layers 203.

In some embodiments, the semiconductor device 200 also includes an innerisolation structure 207 disposed between the gate stacks 205 a and 205b, and a plurality of outer isolation structures 211 disposed on thesides of the gate stacks 205 a and 205 b that are opposite to the sidesof the inner isolation structure 207. In some embodiments, thesemiconductor device 200 further includes a semiconductor material 213disposed over the inner isolation structure 207 and the outer isolationstructures 211, and the portion of the semiconductor material 213between the gate stacks 205 a and 205 b includes an air gap 220. In someembodiments, the air gap 220 is enclosed by the semiconductor material213.

In some embodiments, the top surface of the semiconductor material 213is substantially level with the top surface of the semiconductorsubstrate 201 (i.e., the top surface of the gate stacks 205 a and 205b). Within the context of this disclosure, the word “substantially”means preferably at least 90%, more preferably 95%, even more preferably98%, and most preferably 99%. Moreover, the semiconductor device 200further includes a plurality of inner spacers 209 stacked between thesemiconductor layers (semiconductor substrate 201) of the gate stacks205 a, 205 b and adjoin opposite sides of the gate layers 203.

Still referring to FIG. 2 , the semiconductor device 200 includes aplurality of gate structures 233 disposed over at least one of the gatestacks 205 a and 205 b, and a dielectric layer 235 (also referred to asIMD layer) disposed over the gate stacks 205 a, 205 b and thesemiconductor material 213 and surrounding the gate structures 233, inaccordance with some embodiments. It should be noted that the upperportions of the gate structures 233 are protruded from the dielectriclayer 235. More specifically, the top surfaces and the upper sidewallsof the gate structures 233 are protruded from (i.e., not covered by) thedielectric layer 235.

In some embodiments, the semiconductor device 200 includes a pluralityof landing pads 237 disposed over the gate structures 233, and aplurality of plugs 253 (also referred to as upper plugs) disposed overthe landing pads 237. In some embodiment, the plugs 253 are electricallyconnected to the gate structures 233 through the landing pads 237. Inaddition, the semiconductor device 200 includes a dielectric layer 243(also referred to as IMD layer) disposed over the dielectric layer 235.In some embodiments, the gate structures 233, the landing pads 237 andthe plugs 253 are disposed in the dielectric layer 243. In other words,the gate structures 233, the landing pads 237 and the plugs 253 aresurrounded by the dielectric layer 243.

FIG. 3 is a cross-sectional view illustrating a semiconductor device300, in accordance with some embodiments. In some embodiments, thesemiconductor device 300 includes a semiconductor substrate 301, aplurality of lower plugs 333 and a plurality of barrier layers 331disposed over the semiconductor substrate 301, a plurality of landingpads 337 disposed over the lower plugs 333 and the barrier layers 331,and a plurality of upper plugs 353 disposed over the landing pads 337.In some embodiment, the upper plugs 353 are electrically connected tothe lower plugs 333 through the landing pads 337. In addition, thesemiconductor device 300 includes a dielectric layer 343 (also referredto as IMD layer) disposed over the semiconductor substrate 301.

More specifically, in some embodiments, the lower plugs 333 are disposedover the barrier layers 331, and at least one of the barrier layers 331extends to surround the corresponding lower plugs 333. In other words,the barrier layers 331 cover the bottom surfaces and the sidewalls ofthe lower plugs 333. In addition, at least one of the landing pads 337includes outer portions 337 b and an inner portion 337 a disposedbetween the outer portions 337 b. It should be noted that the innerportions 337 a of the landing pads 337 are disposed over the lower plugs333, and the outer portions 337 b of the landing pads 337 are disposedover the barrier layers 331. In some embodiments, the inner portions 337a are in direct contact with the top surfaces of the lower plugs 333,and the outer portions 337 b are in direct contact with the top surfacesand the upper sidewalls of the barrier layers 331.

In some embodiments, the lower plugs 333, the landing pads 337 and theupper plugs 353 are disposed in the dielectric layer 343. In otherwords, the lower plugs 333, the landing pads 337 and the upper plugs 353are surrounded by the dielectric layer 343. Moreover, the dielectriclayer 343 includes an air gap 350 disposed between the adjacent lowerplugs 333 (i.e., between the adjacent barrier layers 331). In someembodiments, the air gap 350 is enclosed by the dielectric layer 343. Insome embodiments, the upper plugs 353 are separated from the outerportions 337 b of the landing pads 337 by a portion of the dielectriclayer 343. However, in some other embodiments, the upper plugs 353 arein direct contact with the outer portions 337 b of the landing pads 337.

FIG. 4 is a cross-sectional view illustrating a semiconductor device400, in accordance with some embodiments. Similar to the semiconductordevice 200 of FIG. 2 , the semiconductor device 400 includes asemiconductor substrate 401 which has been patterned to form a pluralityof gate stacks 405 a and 405 b. At least one of the gate stacks 405 aand 405 b has a plurality of alternating stacked semiconductor layers(semiconductor substrate 401) and gate layers 403.

In some embodiments, the semiconductor device 400 also includes an innerisolation structure 407 disposed between the gate stacks 405 a and 405b, and a plurality of outer isolation structures 411 disposed on thesides of the gate stacks 405 a and 405 b that are opposite to the sidesof the inner isolation structure 407. In some embodiments, thesemiconductor device 400 further includes a semiconductor material 413disposed over the inner isolation structure 407 and the outer isolationstructures 411, and the portion of the semiconductor material 413between the gate stacks 405 a and 405 b includes an air gap 420. In someembodiments, the air gap 420 is enclosed by the semiconductor material413.

In some embodiments, the top surface of the semiconductor material 413is substantially level with the top surface of the semiconductorsubstrate 401 (i.e., the top surface of the gate stacks 405 a and 405b). Moreover, the semiconductor device 400 further includes a pluralityof inner spacers 409 stacked between the semiconductor layers(semiconductor substrate 401) of the gate stacks 405 a, 405 b and adjoinopposite sides of the gate layers 403.

Still referring to FIG. 4 , the semiconductor device 400 includes aplurality of gate structures 433 and a plurality of barrier layers 431disposed over at least one of the gate stacks 405 a and 405 b, and adielectric layer 435 (also referred to as IMD layer) disposed over thegate stacks 405 a, 405 b and the semiconductor material 413 andsurrounding the gate structures 433, in accordance with someembodiments. More specifically, the gate structures 433 are disposedover the barrier layers 431, and at least one of the barrier layers 431extends to surround the corresponding gate structures 433. In otherwords, the barrier layers 431 cover the bottom surfaces and thesidewalls of the gate structures 433. In addition, it should be notedthat the upper portions of the gate structures 433 and the barrierlayers 431 are protruded from the dielectric layer 435. Morespecifically, the top surfaces and the upper sidewalls of the gatestructures 433 and the top surfaces and the upper sidewalls of thebarrier layers 431 are protruded from (i.e., not covered by) thedielectric layer 435.

In some embodiments, the semiconductor device 400 includes a pluralityof landing pads 437 disposed over the gate structures 433 and thebarrier layers 431, and a plurality of plugs 453 (also referred to asupper plugs) disposed over the landing pads 437. In some embodiment, theplugs 453 are electrically connected to the gate structures 433 throughthe landing pads 437. In some embodiments, at least one of the landingpads 437 includes outer portions 437 b and an inner portion 437 adisposed between the outer portions 437 b. It should be noted that theinner portions 437 a of the landing pads 437 are disposed over the gatestructures 433, and the outer portions 437 b of the landing pads 437 aredisposed over the barrier layers 431. In some embodiments, the innerportions 437 a are in direct contact with the top surfaces of the gatestructures 433, and the outer portions 437 b are in direct contact withthe top surfaces and the upper sidewalls of the barrier layers 431.

In addition, the semiconductor device 400 includes a dielectric layer443 (also referred to as IMD layer) disposed over the dielectric layer435. In some embodiments, the gate structures 433, the landing pads 437and the plugs 453 are disposed in the dielectric layer 443. In otherwords, the gate structures 433, the landing pads 437 and the plugs 453are surrounded by the dielectric layer 443. In some embodiments, theplugs 453 are separated from the outer portions 437 b of the landingpads 437 by a portion of the dielectric layer 443. However, in someother embodiments, the plugs 453 are in direct contact with the outerportions 437 b of the landing pads 437.

FIG. 5 is a flow diagram illustrating a method 10 for forming asemiconductor device (e.g., the semiconductor device 100 of FIG. 1 ),and the method 10 includes steps S11, S13, S15, S17, S19 and S21, inaccordance with some embodiments. The steps S11 to S21 of FIG. 5 areelaborated in connection with the following figures.

FIGS. 6-10 are cross-sectional views illustrating intermediate stagesduring the formation of the semiconductor device 100, in accordance withsome embodiments. As shown in FIG. 6 , the semiconductor substrate 101may be a portion of an integrated circuit (IC) chip that includesvarious passive and active electronic elements, such as resistors,capacitors, inductors, diodes, p-type field effect transistors (pFETs),n-type field effect transistors (nFETs), metal-oxide semiconductor fieldeffect transistors (MOSFETs), complementary metal-oxide semiconductor(CMOS) transistors, bipolar junction transistors (BJTs), laterallydiffused MOS (LDMOS) transistors, high voltage transistors, highfrequency transistors, fin field effect transistors (FinFETs), othersuitable IC components, or combinations thereof.

Depending on the IC fabrication stage, the semiconductor substrate 101may include various material layers (e.g., dielectric layers,semiconductor layers, and/or conductive layers) configured to form ICfeatures (e.g., doped regions, isolation features, gate features,source/drain features, interconnect features, other features, orcombinations thereof). The semiconductor substrate 101 has beensimplified for the sake of clarity. It should be noted that additionalfeatures can be added in the semiconductor substrate 101, and some ofthe features described below can be replaced, modified, or eliminated inother embodiments.

The lower plugs 133 are formed over the semiconductor substrate 101, asshown in FIG. 6 in accordance with some embodiments. The respective stepis illustrated as the step S11 in the method 10 shown in FIG. 5 . Insome embodiments, the lower plugs 133 are made of a conductive material,such as copper (Cu). In some embodiments, at least one of the lowerplugs 133 have a bottom width W1 and a top width W2, and the bottomwidth W1 is greater than the top width W2. In some embodiments, the topportions of the adjacent lower plugs 133 have a distance D1 betweenthem. In some embodiments, the lower plugs 133 have tapered profilestapering away from the semiconductor substrate 101.

Next, a dielectric layer 135 is formed surrounding the lower portions ofthe lower plugs 133 and exposing the upper portions of the lower plugs133, as shown in FIG. 7 in accordance with some embodiments. Therespective step is illustrated as the step S13 in the method 10 shown inFIG. 5 . In some embodiments, the dielectric layer 135 is made ofsilicon oxide, silicon nitride, silicon oxynitride, silicon carbide,silicon oxycarbide, silicon carbonitride, silicon oxycarbonitride, orthe like. The dielectric layer 135 is formed by depositing a dielectricmaterial (not shown) over the semiconductor substrate 101 and the lowerplugs 133 by a deposition process, such as a chemical vapor deposition(CVD) process, an atomic layer deposition (ALD) process, or a physicalvapor deposition (PVD) process, and removing the upper portion of thedielectric layer 135 by a chemical mechanical polishing (CMP) process oran etch-back process.

After the upper portion of the dielectric layer 135 is removed, theupper portions of the lower plugs 133 are protruded from the dielectriclayer 135. More specifically, the top surfaces 133T and the uppersidewalls 133US of the lower plugs 133 are protruded and exposed, whilethe lower sidewalls 133LS of the lower plugs 133 are covered by thedielectric layer 135, in accordance with some embodiments.

Subsequently, a heat treatment process is performed to form the landingpads 137 over the top surfaces 133T and the upper sidewalls 133US of thelower plugs 133, as shown in FIG. 8 in accordance with some embodiments.The respective step is illustrated as the step S15 in the method 10shown in FIG. 5 . In some embodiments, a silicon-containing gas is usedduring the heat treatment process to selectively form the landing pads137 on the exposed top surfaces 133T and the exposed upper sidewalls133US of the lower plugs 133. In some embodiments, the landing pads 137are made of copper germanide (Cu₃Ge). Since the landing pads 137 provideincreased landing areas for the subsequently formed upper plugs to landon, the misalignment issues between the subsequently formed upper plugsand the lower plug 133 may be prevented or reduced.

After the landing pads 137 are formed, the dielectric layer 135 isremoved such that an opening 140 is formed between the adjacent lowerplugs 137, as shown in FIG. 9 in accordance with some embodiments. Therespective step is illustrated as the step S17 in the method 10 shown inFIG. 5 . In some embodiments, the dielectric layer 135 is removed by astripping process (e.g., a wet strip process) and/or an ashing process(e.g., a plasma ashing process). Once the dielectric layer 135 isremoved, the lower sidewalls 133LS of the lower plugs 133 are exposed.

Next, the dielectric layer 143 is deposited over the structure of FIG. 9, and the air gap 150 is formed in the opening 140 between the adjacentlower plugs 133 due to an overhang effect, as shown in FIG. 10 inaccordance with some embodiments. More specifically, the dielectriclayer 143 is deposited over the semiconductor substrate 101, the lowerplugs 133 and the landing pads 137, and the opening 140 (see FIG. 9 ) ispartially filled by the dielectric layer 143. The respective step isillustrated as the step S19 in the method 10 shown in FIG. 5 . In someembodiments, the exposed lower sidewalls 133LS of the lower plugs 133are covered by the dielectric layer 143.

Moreover, in some embodiments, the air gap 150 is formed below thelanding pads 137 and enclosed by the dielectric layer 143. In someembodiments, the top surfaces 133T of the lower plugs 133 are higherthan the topmost surface 150T of the air gap 150. Some materials andprocesses used to form the dielectric layer 143 are similar to, or thesame as those used to form the dielectric layer 135, and details thereofare not repeated herein. Since the lower plugs 133 have the air gap 150between them, the parasitic capacitance between the lower plugs 133 maybe reduced.

Referring back to FIG. 1 , after the dielectric layer 143 is formed, theupper plugs 153 are formed in the dielectric layer 143 and over thelanding pads 137, in accordance with some embodiments. The respectivestep is illustrated as the step S21 in the method 10 shown in FIG. 5 .In some embodiments, the upper plugs 153 are made of a conductivematerial, such as copper (Cu), tungsten (W), aluminum (Al), titanium(Ti), tantalum (Ta), gold (Au), silver (Ag), or a combination thereof.

In addition, the upper plugs 153 may be formed by etching the dielectriclayer 143 to form a plurality of openings (not shown) exposing thecorresponding landing pads 137, depositing a conductive material (notshown) in the openings and over the dielectric layer 143, andplanarizing the conductive material such that the top surfaces of theremaining conductive material (i.e., the upper plugs 153) aresubstantially level with the top surface of the dielectric layer 143.After the upper plugs 153 are formed, the semiconductor device 100 isobtained. Referring to FIGS. 1 and 6 , at least one of the widths W1 andW2 of the lower plug 133 is greater than the width W3 of the upper plug153, and the distance D2 between the adjacent upper plugs 153 is greaterthan the distance D1 between the adjacent lower plugs 133.

FIG. 11 is a flow diagram illustrating a method 30 for forming asemiconductor device (e.g., the semiconductor device 200 of FIG. 2 ),and the method 30 includes steps S31, S33, S35, S37, S39 and S41, inaccordance with some embodiments. The steps S31 to S41 of FIG. 11 areelaborated in connection with the following figures.

FIGS. 12-14 are cross-sectional views illustrating intermediate stagesduring the formation of the semiconductor device 200, in accordance withsome embodiments. As shown in FIG. 12 , the semiconductor substrate 201is patterned to form the gate stacks 205 a and 205 b and the gatestructures 233 are formed over the gate stacks 205 a and 205 b, inaccordance with some embodiments.

The semiconductor substrate 201 may be a semiconductor wafer such as asilicon wafer. Alternatively or additionally, the semiconductorsubstrate 201 may include elementary semiconductor materials, compoundsemiconductor materials, and/or alloy semiconductor materials. Examplesof the elementary semiconductor materials may include, but are notlimited to, crystal silicon, polycrystalline silicon, amorphous silicon,germanium, and/or diamond. Examples of the compound semiconductormaterials may include, but are not limited to, silicon carbide, galliumarsenic, gallium phosphide, indium phosphide, indium arsenide, and/orindium antimonide. Examples of the alloy semiconductor materials mayinclude, but are not limited to, SiGe, GaAsP, AlInAs, AlGaAs, GaInAs,GaInP, and/or GaInAsP.

In some embodiments, the semiconductor substrate 201 includes anepitaxial layer. For example, the semiconductor substrate 201 has anepitaxial layer overlying a bulk semiconductor. In some embodiments, thesemiconductor substrate 201 is a semiconductor-on-insulator substratewhich may include a substrate, a buried oxide layer over the substrate,and a semiconductor layer over the buried oxide layer, such as asilicon-on-insulator (SOI) substrate, a silicon germanium-on-insulator(SGOI) substrate, or a germanium-on-insulator (GOI) substrate.Semiconductor-on-insulator substrates may be fabricated using separationby implantation of oxygen (SIMOX), wafer bonding, and/or otherapplicable methods.

The semiconductor layers (semiconductor substrate 201) and the gatelayers 203 are alternating stacked in the gate stacks 205 a and 205 b,and the inner spacers 209 are formed on opposite sides of the gatelayers 203 and sandwiched between the semiconductor layers(semiconductor substrate 201) in the gate stacks 205 a and 205 b. Insome embodiments, the inner spacers 209 are made of silicon oxide,silicon nitride, or another applicable dielectric material. The innerspacers 209 may be formed by laterally recessing the gate layers 203from sides of the gate layers 203 to form a plurality of gaps betweenthe semiconductor layers (semiconductor substrate 201) of the gatestacks 205 a and 205 b, depositing a dielectric material in the gaps,and removing the excess portions of the dielectric material outside ofthe gaps.

Some materials and the processes used to form the inner isolationstructure 207 and the outer isolation structures 211 are similar tothose used to form the inner spacers 209, and details thereof are notrepeated herein. Moreover, the semiconductor material 213 is formed overthe inner isolation structure 207 and the outer isolation structures211, and an air gap 220 is formed over the inner isolation structure 207due to the high aspect ratio of the opening between the gate stacks 205a and 205 b. In some embodiments, the air gap 220 is enclosed by thesemiconductor material 213.

The semiconductor material 213 may be any suitable material, includingbut not limited to, silicon (Si) or silicon germanium (SiGe). Thesemiconductor material 213 may be formed by a deposition process, suchas a CVD process, an ALD process, or a PVD process. The semiconductormaterial 213 may be formed by a deposition process, and a subsequentplanarization process (e.g., a CMP process). After the semiconductormaterial 213 is formed, the gate structures 233 are formed over the gatestacks 205 a and 205 b. The respective steps are illustrated as thesteps S31 and S33 in the method 30 shown in FIG. 11 .

In some embodiments, the gate structures 233 are made of a conductivematerial, such as copper (Cu). In some embodiments, at least one of thegate structures 233 have a width W4. In some embodiments, the topportions of the adjacent gate structures 233 have a distance D3 betweenthem. It should be noted that since the gate stacks 205 a and 205 b havethe air gap 220 between them, the parasitic capacitance between the gatestacks 205 a and 205 b may be reduced.

Next, the dielectric layer 235 is formed surrounding the lower portionsof the gate structures 233 and exposing the upper portions of the gatestructures 233, as shown in FIG. 13 in accordance with some embodiments.The respective step is illustrated as the step S35 in the method 30shown in FIG. 11 . In some embodiments, the dielectric layer 235 is madeof silicon oxide, silicon nitride, silicon oxynitride, silicon carbide,silicon oxycarbide, silicon carbonitride, silicon oxycarbonitride, orthe like. The dielectric layer 235 is formed by depositing a dielectricmaterial (not shown) over the semiconductor material 213, the gatestacks 205 a, 205 b and the gate structures 233 by a deposition process,such as a CVD process, an ALD process, or a physical vapor depositionPVD process, and removing the upper portion of the dielectric layer 235by a CMP process or an etch-back process.

After the upper portion of the dielectric layer 235 is removed, theupper portions of the gate structures 233 are protruded from thedielectric layer 235. More specifically, the top surfaces 233T and theupper sidewalls 233US of the gate structures 233 are protruded andexposed, while the lower sidewalls 233LS of the gate structures 233 arecovered by the dielectric layer 235, in accordance with someembodiments.

Subsequently, a heat treatment process is performed to form the landingpads 237 over the top surfaces 233T and the upper sidewalls 233US of thegate structures 233, as shown in FIG. 14 in accordance with someembodiments. The respective step is illustrated as the step S37 in themethod 30 shown in FIG. 11 . In some embodiments, a silicon-containinggas is used during the heat treatment process to selectively form thelanding pads 237 on the exposed top surfaces 233T and the exposed uppersidewalls 233US of the gate structures 233. In some embodiments, thelanding pads 237 are made of copper germanide (Cu₃Ge). Since the landingpads 237 provide increased landing areas for the subsequently formedplugs to land on, the misalignment issues between the subsequentlyformed plugs and the gate structures 233 may be prevented or reduced.

Referring back to FIG. 2 , the dielectric layer 243 is formed over thedielectric layer 235 and covering the landing pads 237, and the plugs253 are formed in the dielectric layer 243 and over the landing pads237, in accordance with some embodiments. The respective steps areillustrated as the steps S39 and S41 in the method 30 shown in FIG. 11 .Some materials and processes used to form the dielectric layer 243 aresimilar to, or the same as those used to form the dielectric layer 235,and details thereof are not repeated herein. In some embodiments, theplugs 253 are made of a conductive material, such as copper (Cu),tungsten (W), aluminum (Al), titanium (Ti), tantalum (Ta), gold (Au),silver (Ag), or a combination thereof.

In addition, the plugs 253 may be formed by etching the dielectric layer243 to form a plurality of openings (not shown) exposing thecorresponding landing pads 237, depositing a conductive material (notshown) in the openings and over the dielectric layer 243, andplanarizing the conductive material such that the top surfaces of theremaining conductive material (i.e., the plugs 253) are substantiallylevel with the top surface of the dielectric layer 243. After the plugs253 are formed, the semiconductor device 200 is obtained. Referring toFIGS. 2 and 12 , the width W4 of the gate structure 233 is greater thanthe width W5 of the plug 253, and the distance D4 between the adjacentplugs 253 is greater than the distance D3 between the adjacent gatestacks 205 a and 205 b.

FIG. 15 is a flow diagram illustrating a method 50 for forming asemiconductor device (e.g., the semiconductor device 300 of FIG. 3 ),and the method 50 includes steps S51, S53, S55, S57, S59, S61, S63, S65and S67, in accordance with some embodiments. The steps S51 to S67 ofFIG. 15 are elaborated in connection with the following figures.

FIGS. 16-22 are cross-sectional views illustrating intermediate stagesduring the formation of the semiconductor device 300, in accordance withsome embodiments. As shown in FIG. 16 , a sacrificial dielectric layer323 with openings 330 is formed over the semiconductor substrate 301, inaccordance with some embodiments. In some embodiments, the semiconductorsubstrate 301 is partially exposed by at least one of the openings 330.The respective step is illustrated as the step S51 in the method 50shown in FIG. 15 .

The semiconductor substrate 301 may be similar to the semiconductorsubstrate 101 described above and the description is not repeatedherein. The sacrificial dielectric layer 323 may be made of siliconoxide, silicon nitride, silicon oxynitride, silicon carbide, siliconoxycarbide, silicon carbonitride, silicon oxycarbonitride, or the like.In some embodiments, the sacrificial dielectric layer 323 is formed bydepositing a dielectric material (not shown) over the semiconductorsubstrate 301, and etching the dielectric material by an anisotropicetching process to form the openings 330, such that the openings 330have tapered profiles tapering away from the semiconductor substrate301.

In some other embodiments, the sacrificial dielectric layer 323 isformed by forming sacrificial patterns (not shown) over thesemiconductor substrate 301, forming a sacrificial dielectric material(not shown) surrounding the sacrificial patterns, planarizing thesacrificial dielectric material such that the top surface of theremaining sacrificial dielectric material (i.e., the sacrificialdielectric layer 323) is substantially level with the top surfaces ofthe sacrificial patterns, and removing the sacrificial patterns.

Next, the barrier layers 331 are formed lining the bottom surface andthe sidewalls of the openings 330, and the lower plugs 333 are formedover and surrounded by the barrier layers 331. The respective steps areillustrated as the steps S53 and S55 in the method 50 shown in FIG. 15 .In some embodiments, the barrier layers 331 are made of atitanium-containing material, such as titanium (Ti) or titanium nitride(TiN), and the lower plugs 333 are made of copper (Cu). The barrierlayers 331 and the lower plugs 333 may be formed by depositionprocesses, such as CVD, ALD, PVD, sputtering, or plating. After thedeposition processes, a planarization process (e.g., a CMP process) maybe performed such that the top surfaces of the barrier layers 331 andthe top surfaces of the lower plugs 333 are substantially level with thetop surface of the sacrificial dielectric layer 323.

In some embodiments, at least one of the lower plugs 333 and thecorresponding barrier layers 331 collectively have a bottom width W6 anda top width W7, and the bottom width W6 is greater than the top widthW7. That is, at least one of the lower plugs 333 and the correspondingbarrier layers 331 collectively have tapered profiles tapering away fromthe semiconductor substrate 101, in accordance with some embodiments.Since the thicknesses of the barrier layers 331 are too small comparedto the thicknesses of the lower plugs 333, the bottom width of at leastone of the lower plugs 333 may be substantially the same as the widthW6, and the top width of at least one of the lower plugs 333 may besubstantially the same as the width W7. In other words, the bottomwidths of the lower plugs 333 are greater than the top widths of thelower plugs 333, and at least one of the lower plugs 333 has a taperedprofile tapering away from the semiconductor substrate 101.

In addition, in some embodiments, the top portions of the adjacentbarrier layers 331 have a distance D5 between them. Since thethicknesses of the barrier layers 331 are too small compared to thethicknesses of the lower plugs 333, the distance between the adjacentlower plugs 333 is substantially the same as the distance D5.

After the barrier layers 331 and the lower plugs 333 are formed, thesacrificial dielectric layer 323 is removed, as shown in FIG. 18 inaccordance with some embodiments. The respective step is illustrated asthe step S57 in the method 50 shown in FIG. 15 . In some embodiments,the sacrificial dielectric layer 323 is removed by a stripping process(e.g., a wet strip process) and/or an ashing process (e.g., a plasmaashing process).

Subsequently, the dielectric layer 335 is formed surrounding the lowerportions of the lower plugs 333 and the barrier layers 331 and exposingthe upper portions of the lower plugs 333 and the barrier layers 331, asshown in FIG. 19 in accordance with some embodiments. The respectivestep is illustrated as the step S59 in the method 50 shown in FIG. 15 .Some materials and processes used to form the dielectric layer 335 aresimilar to, or the same as those used to form the sacrificial dielectriclayer 323, and details thereof are not repeated herein. The dielectriclayer 335 is formed by depositing a dielectric material (not shown) overthe semiconductor substrate 301, the lower plugs 333 and the barrierlayers 331 by a deposition process, such as a CVD process, an ALDprocess, or a PVD process, and removing the upper portion of thedielectric layer 335 by a CMP process or an etch-back process.

After the upper portion of the dielectric layer 335 is removed, theupper portions of the lower plugs 333 and the upper portions of thebarrier layers 331 are protruded from the dielectric layer 335. Morespecifically, the top surfaces 333T and the upper sidewalls 333US of thelower plugs 333, and the top surfaces 331T and the upper sidewalls 331USof the barrier layers 331 are protruded (and/or exposed), while thelower sidewalls 333LS of the lower plugs 333 and the lower sidewalls331LS of the barrier layers 331 are covered by the dielectric layer 335,in accordance with some embodiments.

In some embodiments, the formation of the dielectric layer 335 isomitted. In these cases, the sacrificial dielectric layer 323 arepartially removed such that the lower portion of the sacrificialdielectric layer 323 remains covering the lower sidewalls 331LS of thebarrier layers 331 and the lower sidewalls 333LS of the lower plugs 333.

Then, a heat treatment process is performed to form the landing pads 337over the top surfaces 333T and the upper sidewalls 333US of the lowerplugs 333 and over the top surfaces 331T and the upper sidewalls 331USof the barrier layers 331, as shown in FIG. 20 in accordance with someembodiments. The respective step is illustrated as the step S61 in themethod 50 shown in FIG. 15 . In some embodiments, at least one of thelanding pads 337 includes the outer portions 337 b and the inner portion337 a disposed between the outer portions 337 b.

In some embodiments, a silicon-containing gas is used during the heattreatment process to selectively form the inner portions 337 a of thelanding pads 337 on the exposed top surfaces 333T of the lower plugs333, and selectively form the outer portions 337 b of the landing pads337 on the exposed top surfaces 331T and the upper sidewalls 331US ofthe barrier layers 331. In some embodiments, the inner portions 337 aand the outer portions 337 b are made of different materials, but theyare simultaneously formed by the heat treatment process. In someembodiments, the inner portions 337 a are made of tungsten silicide(WSi), and the outer portions 337 b are made of titanium silicide(TiSi). Since the landing pads 337 provide increased landing areas forthe subsequently formed upper plugs to land on, the misalignment issuesbetween the subsequently formed upper plugs and the lower plugs 333 maybe prevented or reduced.

After the landing pads 337 are formed, the dielectric layer 335 isremoved such that an opening 340 is formed between the adjacent lowerplugs 337 (or between the adjacent barrier layers 331 surrounding thelower plugs 337), as shown in FIG. 21 in accordance with someembodiments. The respective step is illustrated as the step S63 in themethod 50 shown in FIG. 15 . In some embodiments, the dielectric layer335 is removed by a stripping process (e.g., a wet strip process) and/oran ashing process (e.g., a plasma ashing process). Once the dielectriclayer 335 is removed, the lower sidewalls 331LS of the barrier layers331 are exposed.

Next, the dielectric layer 343 is deposited over the structure of FIG.21 , and the air gap 350 is formed in the opening 340 between theadjacent lower plugs 333 (or between the adjacent barrier layers 331)due to an overhang effect, as shown in FIG. 22 in accordance with someembodiments. More specifically, the dielectric layer 343 is depositedover the semiconductor substrate 301, the lower plugs 333, the barrierlayers 331 and the landing pads 337, and the opening 340 (see FIG. 21 )is partially filled by the dielectric layer 343. The respective step isillustrated as the step S65 in the method 50 shown in FIG. 15 . In someembodiments, the exposed lower sidewalls 331LS of the barrier layers 331are covered by the dielectric layer 343.

Moreover, in some embodiments, the air gap 350 is formed below thelanding pads 337 and enclosed by the dielectric layer 343. In someembodiments, the top surfaces 333T of the lower plugs 333 are higherthan the topmost surface 350T of the air gap 350. Some materials andprocesses used to form the dielectric layer 343 are similar to, or thesame as those used to form the dielectric layer 335, and details thereofare not repeated herein. Since the lower plugs 333 have the air gap 350between them, the parasitic capacitance between the lower plugs 333 maybe reduced.

Referring back to FIG. 3 , after the dielectric layer 343 is formed, theupper plugs 353 are formed in the dielectric layer 343 and over thelanding pads 337, in accordance with some embodiments. The respectivestep is illustrated as the step S67 in the method 50 shown in FIG. 15 .In some embodiments, the upper plugs 353 are made of a conductivematerial, such as copper (Cu), tungsten (W), aluminum (Al), titanium(Ti), tantalum (Ta), gold (Au), silver (Ag), or a combination thereof.

In addition, the upper plugs 353 may be formed by etching the dielectriclayer 343 to form a plurality of openings (not shown) exposing thecorresponding landing pads 337, depositing a conductive material (notshown) in the openings and over the dielectric layer 343, andplanarizing the conductive material such that the top surfaces of theremaining conductive material (i.e., the upper plugs 353) aresubstantially level with the top surface of the dielectric layer 343.After the upper plugs 353 are formed, the semiconductor device 300 isobtained. Referring to FIGS. 3 and 17 , at least one of the widths W6and W7 is greater than the width W8, and the distance D6 is greater thanthe distance D5.

FIG. 23 is a flow diagram illustrating a method 70 for forming asemiconductor device (e.g., the semiconductor device 400 of FIG. 4 ),and the method 70 includes steps S71, S73, S75, S77, S79, S81, S83, S85and S87, in accordance with some embodiments. The steps S71 to S87 ofFIG. 23 are elaborated in connection with the following figures.

FIGS. 24-28 are cross-sectional views illustrating intermediate stagesduring the formation of the semiconductor device 400, in accordance withsome embodiments. As shown in FIG. 24 , the semiconductor substrate 401,the gate stacks 405 a, 405 b, the gate layers 403, the inner spacers409, the inner isolation structure 407, the outer isolation structures411, and the semiconductor material 413 may be similar to thesemiconductor substrate 201, the gate stacks 205 a, 205 b, the gatelayers 203, the inner spacers 209, the inner isolation structure 207,the outer isolation structures 211, and the semiconductor material 213of the semiconductor device 200, respectively, and the description isnot repeated herein.

Moreover, in some embodiments, the air gap 420 is formed over the innerisolation structure 407 due to the high aspect ratio of the openingbetween the gate stacks 405 a and 405 b. In some embodiments, the airgap 420 is enclosed by the semiconductor material 413. It should benoted that since the gate stacks 405 a and 405 b have the air gap 420between them, the parasitic capacitance between the gate stacks 405 aand 405 b may be reduced.

A sacrificial dielectric layer 423 with openings 430 is formed over thesemiconductor material 413 and the gate stacks 405 a, 405 b, as shown inFIG. 24 in accordance with some embodiments. In some embodiments, atleast one of the gate stacks 405 a and 405 b is partially exposed by thecorresponding openings 430. The respective steps are illustrated as thesteps S71 and S73 in the method 70 shown in FIG. 23 .

The sacrificial dielectric layer 423 may be made of silicon oxide,silicon nitride, silicon oxynitride, silicon carbide, siliconoxycarbide, silicon carbonitride, silicon oxycarbonitride, or the like.In some embodiments, the sacrificial dielectric layer 423 is formed bydepositing a dielectric material (not shown) over the semiconductormaterial 413 and the gate stacks 405 a, 405 b, and etching thedielectric material to form the openings 430. In some other embodiments,the sacrificial dielectric layer 423 is formed by forming sacrificialpatterns (not shown) over the semiconductor material 413 and the gatestacks 405 a, 405 b, forming a sacrificial dielectric material (notshown) surrounding the sacrificial patterns, planarizing the sacrificialdielectric material such that the top surface of the remainingsacrificial dielectric material (i.e., the sacrificial dielectric layer323) is substantially level with the top surfaces of the sacrificialpatterns, and removing the sacrificial patterns.

Next, the barrier layers 431 are formed lining the bottom surface andthe sidewalls of the openings 430, and the gate structures 433 areformed over and surrounded by the barrier layers 431. The respectivesteps are illustrated as the steps S75 and S77 in the method 70 shown inFIG. 23 . In some embodiments, the barrier layers 431 are made of atitanium-containing material, such as titanium (Ti) or titanium nitride(TiN), and the gate structures 433 are made of copper (Cu). The barrierlayers 431 and the gate structures 433 may be formed by depositionprocesses, such as CVD, ALD, PVD, sputtering, or plating. After thedeposition processes, a planarization process (e.g., a CMP process) maybe performed such that the top surfaces of the barrier layers 331 andthe top surfaces of the gate structures 433 are substantially level withthe top surface of the sacrificial dielectric layer 423.

In some embodiments, at least one of the gate structures 433 and thecorresponding barrier layers 431 collectively have a width W9. Since thethicknesses of the barrier layers 431 are too small compared to thethicknesses of the gate structures 433, the width of at least one of thegate structures 433 may be substantially the same as the width W9. Inaddition, in some embodiments, the top portions of the adjacent barrierlayers 431 have a distance D7 between them. Since the thicknesses of thebarrier layers 431 are too small compared to the thicknesses of the gatestructures 433, the distance between the adjacent gate structures 433 issubstantially the same as the distance D7.

After the barrier layers 431 and the gate structures 433 are formed, thesacrificial dielectric layer 423 is removed, as shown in FIG. 26 inaccordance with some embodiments. The respective step is illustrated asthe step S79 in the method 70 shown in FIG. 23 . In some embodiments,the sacrificial dielectric layer 423 is removed by a stripping process(e.g., a wet strip process) and/or an ashing process (e.g., a plasmaashing process).

Subsequently, the dielectric layer 435 is formed surrounding the lowerportions of the gate structures 433 and the barrier layers 431 andexposing the upper portions of the gate structures 433 and the barrierlayers 431, as shown in FIG. 27 in accordance with some embodiments. Therespective step is illustrated as the step S81 in the method 70 shown inFIG. 23 . Some materials and processes used to form the dielectric layer435 are similar to, or the same as those used to form the sacrificialdielectric layer 423, and details thereof are not repeated herein. Thedielectric layer 435 is formed by depositing a dielectric material (notshown) over the semiconductor material 413 and the gate stacks 405 a,405 b, the gate structures 433 and the barrier layers 431 by adeposition process, such as a CVD process, an ALD process, or a PVDprocess, and removing the upper portion of the dielectric layer 435 by aCMP process or an etch-back process.

After the upper portion of the dielectric layer 435 is removed, theupper portions of the gate structures 433 and the upper portions of thebarrier layers 431 are protruded from the dielectric layer 435. Morespecifically, the top surfaces 433T and the upper sidewalls 433US of thegate structures 433, and the top surfaces 431T and the upper sidewalls431US of the barrier layers 431 are protruded (and/or exposed), whilethe lower sidewalls 433LS of the gate structures 433 and the lowersidewalls 431LS of the barrier layers 431 are covered by the dielectriclayer 435, in accordance with some embodiments.

In some embodiments, the formation of the dielectric layer 435 isomitted. In these cases, the sacrificial dielectric layer 423 arepartially removed such that the lower portion of the sacrificialdielectric layer 423 remains covering the lower sidewalls 431LS of thebarrier layers 431 and the lower sidewalls 433LS of the gate structures433.

Then, a heat treatment process is performed to form the landing pads 437over the top surfaces 433T and the upper sidewalls 433US of the gatestructures 433 and over the top surfaces 431T and the upper sidewalls431US of the barrier layers 431, as shown in FIG. 28 in accordance withsome embodiments. The respective step is illustrated as the step S83 inthe method 70 shown in FIG. 23 . In some embodiments, at least one ofthe landing pads 437 includes the outer portions 437 b and the innerportion 437 a disposed between the outer portions 437 b.

In some embodiments, a silicon-containing gas is used during the heattreatment process to selectively form the inner portions 437 a of thelanding pads 437 on the exposed top surfaces 433T of the gate structures433, and selectively form the outer portions 437 b of the landing pads437 on the exposed top surfaces 431T and the upper sidewalls 431US ofthe barrier layers 431. In some embodiments, the inner portions 437 aand the outer portions 437 b are made of different materials, but theyare simultaneously formed by the heat treatment process. In someembodiments, the inner portions 437 a are made of tungsten silicide(WSi), and the outer portions 437 b are made of titanium silicide(TiSi). Since the landing pads 437 provide increased landing areas forthe subsequently formed plugs to land on, the misalignment issuesbetween the subsequently formed plugs and the gate structures 433 may beprevented or reduced.

Referring back to FIG. 4 , the dielectric layer 443 is formed over thedielectric layer 435 and covering the landing pads 437, and the plugs453 are formed in the dielectric layer 443 and over the landing pads437, in accordance with some embodiments. The respective steps areillustrated as the steps S85 and S87 in the method 70 shown in FIG. 23 .Some materials and processes used to form the dielectric layer 443 aresimilar to, or the same as those used to form the dielectric layer 435,and details thereof are not repeated herein. In some embodiments, theplugs 453 are made of a conductive material, such as copper (Cu),tungsten (W), aluminum (Al), titanium (Ti), tantalum (Ta), gold (Au),silver (Ag), or a combination thereof.

In addition, the plugs 453 may be formed by etching the dielectric layer443 to form a plurality of openings (not shown) exposing thecorresponding landing pads 437, depositing a conductive material (notshown) in the openings and over the dielectric layer 443, andplanarizing the conductive material such that the top surfaces of theremaining conductive material (i.e., the plugs 453) are substantiallylevel with the top surface of the dielectric layer 443. After the plugs453 are formed, the semiconductor device 400 is obtained. Referring toFIGS. 4 and 25 , the width W9 is greater than the width W10, and thedistance D8 is greater than the distance D7.

Embodiments of the semiconductor devices 100, 200, 300 and 400 andmethod for forming the same are provided in the disclosure. In someembodiments, at least one of the semiconductor devices 100, 200, 300 and400 includes a plurality of conductive features (e.g., the lower plugs133 of the semiconductor device 100, the gate structures 233 of thesemiconductor device 200, the lower plugs 333 of the semiconductordevice 300, and the gate structures 433 of the semiconductor device400), a plurality of landing pads (e.g., the landing pads 137 of thesemiconductor device 100, the landing pads 237 of the semiconductordevice 200, the landing pads 337 of the semiconductor device 300, andthe landing pads 437 of the semiconductor device 400) disposed over thetop surfaces and the upper sidewalls of the conductive features, and aplurality of upper plugs (e.g., the upper plugs 153 of the semiconductordevice 100, the plugs 253 of the semiconductor device 200, the upperplugs 353 of the semiconductor device 300, and the plugs 453 of thesemiconductor device 400) disposed over the landing pads. The landingpads provide increased landing areas for the upper plugs to land on.Therefore, the misalignment issues between the upper plugs and theconductive features thereunder may be prevented or reduced, and thecontact resistances may be reduced.

Moreover, air gaps are formed below the landing pads and between theconductive features (or between the gate stacks under the conductivefeatures). For example, the air gap 150 of the semiconductor device 100is formed between the lower plugs 133, the air gap 220 of thesemiconductor device 200 is formed between the gate stacks 205 a and 205b, the air gap 350 of the semiconductor device 300 is formed between thelower plugs 333, and the air gap 420 of the semiconductor device 400 isformed between the gate stacks 405 a and 405 b. Therefore, the parasiticcapacitances between the conductive features or the gate stacks may bereduced. As a result, the yield rate of the semiconductor devices 100,200, 300 and 400 may be increased, and the overall device performancemay be improved.

In one embodiment of the present disclosure, a semiconductor device isprovided. The semiconductor device includes a first lower plug and asecond lower plug disposed over a semiconductor substrate. Thesemiconductor device also includes a first landing pad disposed over atop surface and upper sidewalls of the first lower plug, and a firstupper plug disposed over the first landing pad and electricallyconnected to the first lower plug. A width of the first lower plug isgreater than a width of the first upper plug. The semiconductor devicefurther includes a dielectric layer disposed over the semiconductorsubstrate. The first lower plug, the second lower plug, the firstlanding pad and the first upper plug are disposed in the dielectriclayer, and the dielectric layer includes an air gap disposed between thefirst lower plug and the second lower plug.

In another embodiment of the present disclosure, a semiconductor deviceis provided. The semiconductor device includes a first gate stack and asecond gate stack disposed over a semiconductor substrate. At least oneof the first gate stack and the second gate stack includes a pluralityof gate layers, and the first gate stack and the second gate stack havean air gap therebetween. The semiconductor device also includes a firstgate structure and a second gate structure disposed over the first gatestack and the second gate stack, respectively, and a first dielectriclayer surrounding lower sidewalls of the first gate structure and lowersidewalls of the second gate structure. The semiconductor device furtherincludes a first landing pad disposed over a top surface and uppersidewalls of the first gate structure, and a first plug disposed overthe first landing pad and electrically connected to the first gatestructure. A width of the first gate structure is greater than a widthof the first plug. In addition, the semiconductor device includes asecond dielectric layer disposed over the first dielectric layer. Thefirst landing pad and the first plug are surrounded by the seconddielectric layer.

In another embodiment of the present disclosure, a method for forming asemiconductor device is provided. The method includes forming a firstlower plug and a second lower plug over a semiconductor substrate, andforming a first dielectric layer surrounding the first lower plug andthe second lower plug. A top surface and upper sidewalls of the firstlower plug and a top surface and upper sidewalls of the second lowerplug are protruded from the first dielectric layer. The method alsoincludes performing a heat treatment process to form a first landing padover the top surface and the upper sidewalls of the first lower plug anda second landing pad over the top surface and the upper sidewalls of thesecond lower plug, and removing the first dielectric layer after theheat treatment process is performed such that an opening is formedbetween the first lower plug and the second lower plug. The methodfurther includes depositing a second dielectric layer in the opening andover the first landing pad and the second landing pad such that an airgap is formed in the opening and enclosed by the second dielectriclayer, and forming a first upper plug in the second dielectric layer andover the first landing pad. A width of the first lower plug is greaterthan a width of the first upper plug.

In another embodiment of the present disclosure, a method for forming asemiconductor device is provided. The method includes forming a firstgate stack and a second gate stack over a semiconductor substrate. Atleast one of the first gate stack and the second gate stack includes aplurality of gate layers, and the first gate stack and the second gatestack have an air gap therebetween. The method also includes forming afirst gate structure and a second gate structure over the first gatestack and the second gate stack, respectively, and forming a firstdielectric layer surrounding the first gate structure and the secondgate structure. A top surface and upper sidewalls of the first gatestructure and a top surface and upper sidewalls of the second gatestructure are protruded from the first dielectric layer. The methodfurther includes performing a heat treatment process to form a firstlanding pad over the top surface and the upper sidewalls of the firstgate structure and a second landing pad over the top surface and theupper sidewalls of the second gate structure. In addition, the methodincludes forming a second dielectric layer covering the first landingpad and the second landing pad, and forming a first plug in the seconddielectric layer and over the first landing pad. A width of the firstgate structure is greater than a width of the first plug.

The embodiments of the present disclosure have some advantageousfeatures. By forming a landing pad between a lower plug and an upperplug (or a gate structure and a plug over the gate structure), themisalignment issues may be prevented or reduced, which reduces thecontact resistance. Moreover, by forming an air gap below the landingpad and between the adjacent lower plugs (or the adjacent gate stacks),the parasitic capacitance may be reduced. As a result, the yield rate ofthe semiconductor device may be increased, and the device performancemay be improved.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. For example,many of the processes discussed above can be implemented in differentmethodologies and replaced by other processes, or a combination thereof.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present disclosure. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, and steps.

What is claimed is:
 1. A semiconductor device, comprising: a first gatestack and a second gate stack disposed over a semiconductor substrate,wherein at least one of the first gate stack and the second gate stackcomprises a plurality of gate layers, and wherein the first gate stackand the second gate stack have an air gap therebetween; a first gatestructure and a second gate structure disposed over the first gate stackand the second gate stack, respectively; a first dielectric layersurrounding lower sidewalls of the first gate structure and lowersidewalls of the second gate structure; a first landing pad disposedover a top surface and upper sidewalls of the first gate structure; afirst plug disposed over the first landing pad and electricallyconnected to the first gate structure, wherein a width of the first gatestructure is greater than a width of the first plug; and a seconddielectric layer disposed over the first dielectric layer, wherein thefirst landing pad and the first plug are surrounded by the seconddielectric layer; a first barrier layer covering a bottom surface, thelower sidewalls and the upper sidewalls of the first gate structure,wherein a portion of the first barrier layer is sandwiched between thefirst landing pad and the first gate structure.
 2. The semiconductordevice of claim 1, further comprising: a second landing pad disposedover a top surface and upper sidewalls of the second gate structure; anda second plug disposed over the second landing pad and electricallyconnected to the second gate structure, wherein a distance between thefirst plug and the second plug is greater than a distance between thefirst gate stack and the second gate stack.
 3. The semiconductor deviceof claim 1, wherein the first landing pad is in direct contact with thefirst dielectric layer.
 4. The semiconductor device of claim 1, furthercomprising: a semiconductor material disposed between the first gatestack and the second gate stack, wherein the air gap is enclosed by thesemiconductor material.
 5. The semiconductor device of claim 1, whereinthe first landing pad is made of copper germanide (Cu₃Ge).
 6. Thesemiconductor device of claim 1, wherein the first landing padcomprises: an inner portion covering the top surface of the first gatestructure; and a plurality of outer portions covering top surfaces andupper sidewalls of the first barrier layer, wherein the inner portionand the outer portions are made of different materials.
 7. Thesemiconductor device of claim 6, wherein top surfaces of the outerportions of the first landing pad are higher than a top surface of theinner portions of the first landing pad.
 8. The semiconductor device ofclaim 6, wherein the inner portion of the first landing pad is made oftungsten silicide (WSi), and the outer portions of the first landing padare made of titanium silicide (Ti Si).
 9. A method for forming asemiconductor device, comprising: forming a first gate stack and asecond gate stack over a semiconductor substrate, wherein at least oneof the first gate stack and the second gate stack comprises a pluralityof gate layers, and wherein the first gate stack and the second gatestack have an air gap therebetween; forming a first gate structure and asecond gate structure over the first gate stack and the second gatestack, respectively; forming a first dielectric layer surrounding thefirst gate structure and the second gate structure, wherein a topsurface and upper sidewalls of the first gate structure and a topsurface and upper sidewalls of the second gate structure are protrudedfrom the first dielectric layer; performing a heat treatment process toform a first landing pad over the top surface and the upper sidewalls ofthe first gate structure and a second landing pad over the top surfaceand the upper sidewalls of the second gate structure; forming a seconddielectric layer covering the first landing pad and the second landingpad; forming a first plug in the second dielectric layer and over thefirst landing pad, wherein a width of the first gate structure isgreater than a width of the first plug; forming a sacrificial dielectriclayer with a first opening and a second opening over the first gatestack and the second gate stack; forming a first barrier layer and asecond barrier layer lining the first opening and the second opening,respectively; forming the first gate structure and the second gatestructure over the first barrier layer and the second barrier layer,respectively; and removing the sacrificial dielectric layer after thefirst gate structure and the second gate structure are formed.
 10. Themethod for forming a semiconductor device of claim 9, furthercomprising: forming a second plug in the second dielectric layer andover the second landing pad, wherein a distance between the first plugand the second plug is greater than a distance between the first gatestack and the second gate stack.
 11. The method for forming asemiconductor device of claim 9, wherein a silicon-containing gas isused during the heat treatment process.
 12. The method for forming asemiconductor device of claim 9, wherein the first landing padcomprises: a first inner portion covering the top surface of the firstgate structure; and a plurality of outer portions covering top surfacesand upper sidewalls of the first barrier layer, wherein top surfaces ofthe outer portions are higher than a top surface of the inner portion,and wherein the inner portion and the outer portions are formedsimultaneously by the heat treatment process.